Speed indicator



y 8 93 v B. P. ROMAIN I I ,7 SPEED INDICATOR Filed Dec. 11, 1929' x Patented July 28, 1931 1 UNITED STATES PATENT on-"Ice BURCHARD P. ROMAIN, OF SOUTH ORANGE, EW JERSEY, ASSIGNOR TO WESTON ELEC- TRICAL INSTRUMENT CORPORATION, OF NEWARK, NEW JERSEY, A CORPORATION OF NEW JERSEY SPEED INDICATOR Application filed December 11, 1929. Serial No. 413,386.

This invention relates tospeed indicators of the type including a generator and a voltmeter, and particularly to a method of and g age decreases as the temperature of the magneto rises. This decrease in voltage, for any given speed, is due to two causes. In the first place, the magnetic field established by the permanent magnet decreases as the temperature increases, and, in the second place, the voltage drop in the armature winding increases due to its increased. resistance at a higher temperature.

An object of the present invention is to provide a speed indicator in which the terminal voltage is substantially independent of the temperature of the magneto. F urther objects of the invention are to provide a method of and circuit arrangements for rendering the terminal voltage of the generator substantially. independent of those changes in generated voltage which arise from the varying temperature of the magneto.

More specifically, an object of the invention is to provide a speed indicator including a magneto which has shunted across the terminals thereof a resistance .Whose eflective value increases with temperature at such a rate that the terminal voltage is substantially independent of temperature changes.

These and other objects of the invention will be apparent from the following specification, when taken with the accompanying drawings, in which:

Fig. 1 is a'plan view of a speed indicator, the cover plate of the generator being omitted and the circuit elements embodying the present invention being shown diagrammatically;

.4, that contact with a commutator 5.

Fig. 2 is a diagrammatic illustration of the circuits and general arrangement of the speedometer.

In Fig. 1, the reference character G indicates a magneto generator of the form described in the copending application of Edward F. Weston, Ser. No. 366,488, filed May 27, 1929.

As described in the said application, the

generator includes a brush holder 1 pivotally mounted on the pin 2 and carrying brushes 3, By means oi a flexible cable 6, the terminals 7, 8, of the generator are connected with a voltmeter V.

In accordance with the present invention, a spool or form 9 is mounted between the terminals and carries a pair of resistors R R that are, respectively, in series with the armature winding A and in shunt across the generator terminals. The resistor R is formed of material such as manganin having a small resistance temperature coefficient and the resistor R is formed of a material, such ascopper or nickel, having a high resistance temperature coeflicient.

Vhen the temperature increases, the weakening of the magnetic field and the increased resistance of the armature winding A tend to decrease the terminal voltage, but the increased resistance of the resistor R decreases the current through it and hence decreases the resistance drop in armature winding A and resistor R due to that portion of the totalmagneto current which flows through the shunt resistor R The values of the various resistances in the circuit may be so chosen that substantially perfect temperature compensation can be secured. If the armature winding A has a materially lower resistance temperature coefficient than the shunt resistor R it is possible to omit the series resistor B In general, however, greater freedom of design ispossible when the series circuit between the generator terminals includes a resistor R in addition to the resistance provided by the armature winding. I v

In one'particular case, and for use with a voltmeter having a total resistance of 900 ohms, and giving a full scale deflection with volts applied across its terminals, the resistances included 111 the generator elements were:

Ohms

R manganin resistor 106 R nickel wire, at 20 C 900 A, copper wire, at 20 C 44 Resistance temperature ooellicient of shunt R ier degree C 0.00sl6l The scale of the voltmeter V was graduated from zero to 1250 revolutions per minute, and at 1250 R. P. M., the open circuit voltage generated by the armature was 0 volts. A test of this speed indicator showed an error of but 0.25% for a temperature variation from 35 C. to +-1l5 C., i. e., a total temperature change of 80 C. A similar magneto without the temperature compensating resistors R and R would show an error of approximately 2.2% for the same ten'iperature change.

will be apparent that the invention is not limited to the particular values given in the above example, or to the provision of compensating resistors that suppress temperature variations of any particular range. The invention may be employed with either alternating or direct current magnetos, and the physical and electrical design of the magneto may be quite different from that embodied in the magneto which is shown in Fig. 1.

I claim:

1. In a speed indicator of the type including a generator and a voltmeter, a generator comprising a permanent magnet, an armature revoluble in the field established by said magnet, a winding on said armature, a pair of terminals for said generator, circuit elelncnts including brushes for connecting said winding between said terminals, and a resistor shunted across said terminals, the resistance-temperture coelficient of said resistor being higher than the resistance-temperature coetlicient of the generator series circuit between said terminals.

2. The invention as set forth in claim 1, wherein the generator series circuit between said terminals includes a resistor in series with the armature winding, the resistancetemperatu're coefiicient of said series resistor being lower than that or" said shunt resistor.

3. The invention as set forth in claim 1, wherein the armature winding is formed of copper wire, and the generator series circuit between said terminals includes a resistor having a substantially zero resistance-temperature coefiicient.

l. A generator comprising the combination with means establishing a magnetic field, an armature revoluble in said field, a winding on said armature, and generator output terminals, of means rendering the voltage across said terminals substantially independent of changes in the temperature of said generators; said last-mentioned means comprising a resistor serially connected between said armature winding and said terminals, and a second resistor connected between said terminals, said second resistor having a higher temperattire-resistance coellicient than that of the armature winding circuit between said terminals.

The invention as set forth in claim 4, wherein said armature winding comprises copper wire, said first resisto is a manganin wire having an approximately zero temperature-resistance coellicient, and said second resistor is a nickel wire.

In testimony whereof, I atlix my signature.

BUROHARD P. tOMAIN. 

